A small Rockville computer manufacturer, Denro Laboratories Inc., took a big gamble: It decided to put all its money on one race and to take on the guys with the biggest of the big bucks. And it won.

After concentrating its efforts since 1966 on the development of air traffic communications control equipment, Denro recently beat out AT&T, Western Union, Westinghouse, Litton Industries and CP National Corp. of California in a bid for a $34.7 million Federal Aviation Administration contract to make microprocessor radio equipment for about 53 airports around the country.

Denro Vice President Edward C. Hanna Jr. attributes the company's success to its specialization in the air traffic control field.

"Having the superior technical proposal for this equipment against Westinghouse and Litton," Hanna said, "demonstrates that a small group of engineers that are solely concerned with air traffic control can produce a better product than a large company that builds large computers and then tries to adapt them to an air traffic control environment."

Hanna said the equipment Denro will develop for the FAA will consist of "state of the art" microprocessors to be used by air traffic controllers in communicating on radio frequencies.

"The unique thing is the communication in one facility is all controlled by microprocessors," he said. "There's no single component that would put the entire system down.

Denro will hire only about 40 additional employes as a result of the contract award because, according to Hanna, the FAA equipment is "parts-intensive" rather than "labor-intensive." The digital engineers Denro plans to hire will supplement the current staff of 180 people.

Hanna said the contract calls for equipment for a minimum of 53 air traffic control systems, including a unit for the Washington National Airport, to be leased with an option to buy. Installation and maintenance will be subcontracted to Avco Corp., Hanna said.

Denro Laboratories is a wholly-owned subsidiary of First General Resources Co., which Hanna described as a holding company in New York that also owns 55 percent of an insurance company and several automobile rental franchises. Denro has previously had average annual sales of $4 million, according to Hanna.